Monday, August 11, 2014

Hume, Smith, and the Myth of Mirror Neurons

For both David Hume and Adam Smith, sympathy or "fellow-feeling with any passion whatever" is the psychological ground of our moral experience.  From his reading of Hume and Smith, Charles Darwin adopted this idea of sympathy in explaining the evolution of human morality.

Hume and Smith explained sympathy as a process of "mirroring."  "In general we may remark," Hume observed, "that the minds of men are mirrors to one another, not only because they reflect each others emotions, but also because those rays of passions, sentiments and opinions may be often reverberated, and may decay by insensible degrees."

In 1996, some Italian neuroscientists reported a remarkable discovery in their study of neural activity in the brains of macaque monkeys.  They found neurons that would fire when the monkeys executed an action (like picking up a raisin with their hands) or when observing the same action executed by someone else (as when the human experimenters picked up a raisin with their hands).  They called these "mirror neurons," and they identified them as the neural mechanism by which both monkeys and human beings understand the actions of others by simulating those actions within their own minds.

Since 2006, I have written a series of posts--here, here, here, and  here--suggesting that this theory of mirror neurons provides a neuroscientific confirmation for this Humean, Smithian, and Darwinian account of sympathy as the natural ground of morality and social life generally.  Other commentators have made the same suggestion.  For example, Russell Hardin has done this in his account of "mirroring" in Hume's moral psychology in David Hume: Moral & Political Theorist (Oxford University Press, 2007) (pp. 41-45).

In recent years, however, I have become increasingly suspicious of the mirror neuron theory as the claims of its scientific proponents have become ever more exaggerated and as these exaggerated claims have entered into popular culture.  This has become the new theory of everything about the mind, because it has been invoked to explain everything from empathy and language to stuttering and business leadership.

So I was pleased to see Gregory Hickok's new book--The Myth of Mirror Neurons: The Real Neuroscience of Communication and Cognition (Norton, 2014)--which surveys the theoretical and empirical weaknesses of the mirror neuron theory.  Hickok is a neuroscientist at the University of California, Irvine.  (Hickok has a blog where he comments on some of his research.) 

As he has indicated in a short essay in The New York Times, the myth of mirror neurons follows the pattern of other myths about the brain.  First, there's an interesting discovery.  Then, this discovery is expanded into a grand theory of the human mind.  Finally, this extravagant theory enters pop culture so that people casually appeal to it to explain more and more of human psychology without any awareness of how dubious the theory has become.  In addition to the myth of mirror neurons, Hickok identifies three other such myths about the brain:  the belief that we only use 10 percent of our brain (see the movie "Lucy"), the belief in the left brain/right brain dichotomy, and the belief that mapping the circuitry of the brain will give us a complete understanding of the mind.

Hickok does not doubt that mirror neurons exist in the brains of monkeys and human beings.  But he does doubt that mirror neurons alone can explain how an animal understands the meaning of the actions of other animals.  The great appeal of the mirror neuron theory is that it's so simple in explaining the complexity of the mind.  But that's just the problem--it's too simple.  The system of mirror neurons does whatever it does only in complex interactions with other neural systems throughout the brain.

Hickok points out many problems with the mirror neuron theory.  First of all, we can understand many actions that we ourselves cannot perform.  Smith observed that men can sympathize with women's labor during childbirth despite the fact that men have no direct experience of that themselves.  Moreover, Hickok points out that people suffering disorders of the motor system that make it impossible for them to move can still understand action.  People with damage to motor speech centers cannot speak, but they can still understand the speech of others.  Individuals with cerebral palsy who cannot speak or control their bodies can still understand human social life.  Most dramatically, Christopher Nolan became a successful novelist--showing an acute understanding of human life--although he had been quadriplegic from birth.

Apparently, people like Nolan can gain a conceptual ability for understanding actions that they themselves cannot execute.  This indicates that understanding action must depend on conceptual reasoning that goes beyond physical enactment of the action.  As Hume and Smith recognize, a sympathetic understanding of what others are experiencing requires a conceptual projection of oneself through imagination.

Information is certainly stored in the motor system of the brain, but this is motoric information and not meaningful (semantic) information.  This is indicated by the fact that people who suffer "semantic dementia"--who cannot name objects or understand certain words--have suffered damage to parts of the temporal lobes that do not involve the motor system.

Understanding actions is a complicated cognitive activity that arises from the interaction of many neural systems and not just the motor system.  We can reach this general conclusion even though no one understands yet how exactly this happens in the brain.

Hume, Smith, and Darwin were right to see that social understanding and moral judgment require that the brain mirror or simulate the experiences of others, but this cannot be reduced to a simple activity of the motor system, and the complexity of the neural processes by which the brain does this is not yet fully explained.

In his review of Hickok's book, Christian Keysers complains that Hickok is attacking a straw man ("The Straw Man in the Brain," Science 343 [16 January 2015]: 240).  Hickok rightly disputes the exaggerated claims about mirror neurons made in popular culture, Keysers observes, but these exaggerated claims are not made by the scientists engaged in studying mirror neurons.  Giacomo Rizzolatti and others in this field of research have conceded that mirror neurons are not sufficient for language or action understanding, because mirror neurons are only part of a complex circuitry that is not yet fully understood.

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